The present invention pertains to apparatus for fragmenting kidney stones and other bodily concretions and, more particularly, to apparatus for nonsurgically disintegrating concretions by means of a focused shock wave.
It is estimated that kidney stone disease will affect one percent of the American population at some time in their lives. The disease results from calcium deposits which coagulate into stones (calculi) within the kidney. These stones may then block the ureter causing infection and possible kidney failure. Often, the stones are sufficiently small such that they spontaneously pass through the ureters with varying degree of discomfort. If a stone exceeds one centimeter in diameter, however, in all likelihood it will be too large for passage through the ureter, whereby treatment is required.
Heretofore, the principal method for treating kidney and bladder stones has been by surgical removal.
Kidney stones are commonly composed of calcium compounds and are brittle, comparable to porous ceramic with a tensile strength of approximately 1,000 psi. It is notable that the tensile strength of a stone is approximately one-eighth its compressive strength. This brittle characteristic of the renal calculi has led to the development of apparatus for disintegrating the stones within the body, allowing the fragments to pass from the body during normal elimination.
In one such approach, a lithotriptor is inserted through the urethra into the interior of the body and is positioned into abutting relationship with the bladder or kidney stone to be fragmented. The lithotriptor is connected to a membrane which forms the closing portion of a fluid filled chamber. A high energy spark discharge within the fluid filled chamber creates a shock wave which, when transmitted through the membrane to the lithotriptor, imparts a tensile stress on the stone causing it to shatter.
Although some successful lithotripsy treatments have been reported, there has also been an incidence of bladder wall perforations and shocks to the operators from use of the high voltage equipment. In addition, inasmuch as the procedure is invasive, there are attendant risks involved.
In another approach, the concretions are shattered nonsurgically by use of a shock wave. If a short pressure pulse, such as a shock front, is applied to a stone, the wave will traverse the stone and reflect off the stone/tissue boundary creating a tension stress wave. Due to the brittle nature of the stone, the stress wave, if of sufficient magnitude, will result in stone fragmentation. If the transit time across the stone is less than the pulse width, the momentum in the shock wave is transferred to the stone as a nonfragmenting acceleration. It is essential, therefore, that the shock wave have a controlled, short duration.
In this procedure, the shock wave is focused on the stone by the use of an ellipsoidal waveguide. The waveguide is positioned against the external tissue of the patient's body such that one focal point of the waveguide is coincident with the stone to be treated. Orthogonally positioned X-ray cameras are used to assure proper positioning. The waveguide is filled with a fluid, such as water, and electrodes are positioned at the other ellipsoid focal point. A high energy spark discharged across the electrodes produces a resultant shock wave in the fluid. This shock is reflected off the surfaces of the reflector and through the liquid medium and body tissue to the other reflector focal point, thereby fracturing or fragmenting the stone. The process is repeated until the fragments are sufficiently small such that they may be passed by the body in the normal manner.
Initial studies have indicated that the shock wave employed does not result in tissue or bone damage.
Whereas the shock wave treatment for renal calculi avoids risks incident to surgery, it nonetheless has attendant risks of its own. First, there is the possibility that a current will be passed through the patient at the time of spark discharge and disturb his cardiac pattern. Further, potentially lethal high voltage equipment must be employed to produce the spark discharge. This poses risks to both the patient and the equipment operator. In addition, the spark gap electrodes for use in the aforementioned technique exhibit a short lifetime, resulting in frequent replacement of the electrodes and resultant high cost due to the relatively expensive electrodes which must be employed.